It is known to use ceramic exhaust port liners in the exhaust channels of cylinder heads for internal combustion engines, especially for engines equipped with a turbocharger. The exhaust port liners minimize loss of heat from the exhaust gases and thereby provide the turbocharger with hotter gases to enhance its operation.
It is also known to line exhaust manifolds, pipes and mufflers with a ceramic liner to retain the high temperature of the exhaust gases longer for improved secondary combustion of potentially polluting hydrocarbons present in the exhaust gases.
A typical approach to the manufacture of products such as cylinder heads and exhaust manifolds having ceramic liners therein has involved fabricating a hollow liner of a suitable ceramic material, filling the hollow liner with a mixture of foundry sand (e.g., silica sand) and a resin or other binder which cures or hardens to form a bonded core in the liner, placing one or more of the cored liners in a mold cavity of a casting mold (e.g., a permanent mold or a sand mold), casting molten iron or aluminum into the mold cavity about the cored liners, removing the casting from the mold after the molten metal solidifies and then removing the bonded cores from inside the cast in-situ ceramic liner by suitable mechanical or chemical means. A product, such as a cylinder head or exhaust manifold, is thereby cast with one or more ceramic liners cast in-situ therein.
When this approach has been applied to the manufacture of cylinder heads having cast in-situ ceramic exhaust port liners, the ceramic liners have been observed to be susceptible to cracking during the casting process as a result of differential thermal expansion differences between the ceramic liner and the bonded core formed therein. In particular, when molten metal is cast about the cored exhaust port liner, the liner and core can be heated to a sufficiently high temperature that tensile stresses are exerted on the liner by the bonded core which expands at a greater rate. This thermal cracking problem is exacerbated by the brittleness, low tensile strength and low thermal expansion coefficient of the ceramic materials used to make the exhaust port liners.
The degree of susceptibility of the ceramic liner to cracking depends on the configuration of the liner, the design of the cylinder head cast therearound as well as the process used to cast the cylinder head. Configurational and design modifications of the exhaust port liners can sometimes be made to lessen or reduce their susceptability to cracking, but these modifications typically increase the cost and the complexity of the liners.
It is an object of the invention to provide a method of making cylinder heads with ceramic exhaust port liners cast in-situ therein wherein differential thermal expansion/contraction stress-induced breakage or cracking of the ceramic exhaust port liners is reduced or minimized regardless of the type (configuration) of the liners employed.